1. Field of the Invention
The present invention relates to a solid electrolytic capacitor and a method of manufacturing the solid electrolytic capacitor.
2. Description of the Related Art
Recently, a solder including no lead is being used in a reflow process in consideration of environmental problem. The solder has a melting point higher than that of a solder including lead. It is therefore necessary to increase temperature of the reflow compared to that of conventional reflow. This may result in increase of thermal stress applied to a solid electrolytic capacitor, swelling of the capacitor, and degradation of the capacitor.
Japanese Patent Application Publication No. 11-204377 (hereinafter referred to as Document 1) discloses an art where a polymerized element is protected by a thermoset resin, in order to restrain the swelling. Japanese Patent Application Publication No. 2000-58389 (hereinafter referred to as Document 2) discloses an art where an element is subjected to a thermal treatment of 200 degrees C. to 300 degrees C. before being housed in a case. Japanese Patent Application Publication No. 2001-284179 (hereinafter referred to as Document 3) discloses an art where an element in which a vinylon separator is included with a silver coated lead line is subjected to a thermal treatment of 175 degrees C. to 300 degrees C. before monomer oxidizer is impregnated into the element. Japanese Patent Application Publication No. 2002-110466 (hereinafter referred to as Document 4) discloses an art where ferric p-toluene sulfonate solution is impregnated into an element having a vinylon separator and the element is subjected to a thermal treatment of 150 degrees C. to 200 degrees C.
However, the polymerized element is subjected to a stress because of hardening of the thermoset resin in a case where the thermoset resin protects the polymerized element, with the art disclosed in Document 1. This results in increase of leaking current. An anode oxide coating is subjected to a thermal stress because of the treatment, with the art disclosed in Document 2. This results in increase of the leaking current. The silver coating is sulfurized into a silver sulfide because of sulfidizing gas generated during chemical polymerization of monomer and oxidizer, with the art disclosed in Document 3. This results in a barrier during soldering. And vinylon fiber used for the vinylon separator is not generally used. A separator made of only vinylon fiber is expensive and has a high density. It is difficult to provide an inexpensive capacitor having high ESR, with the art disclosed in Document 4.
And so, a thermal degradation may be used in order to restrain swelling caused by the reflow, because oxidizer degrades natural fiber after the polymerization of monomer. However, the anode oxide coating may be degraded because of thermal stress, and the leaking current may be increased.
A method using synthesized fiber separator may be used in order to restrain swelling caused by reflow heat. However, the synthesized fiber separator is not generally used and is expensive. It is therefore difficult to provide an inexpensive capacitor. And it is difficult to reduce the density of the synthesized separator compared to a natural fiber separator. Therefore, the ESR may be degraded, compared to a case where the natural fiber separator is used. When a rolled element is fabricated with a low density separator manufacturable with present technology, strength of the separator may not be sufficient. In this case, the separator may be broken during rolling.